CT scanning is a relatively young but well-known technology. In simple terms, CT scanning is a nondestructive or noninvasive method of generating sectional views of an object. The concept of CT scanning has been applied to industrial processes, flaw detection and geological sub-strata analysis, for example. Medical CT scanning has also been found to be very useful. The medical community has adopted the technique for obvious reasons, and medical applications will be emphasized herein, although the present invention could be used in conjunction with any CT system irrespective of the nature of the test subject.
Most CT systems include a radiation source, a radiation detector and means for interpreting and displaying the data received by the detector. In medical CT scanners, the patient is typically positioned on a table between the source and detector, wherein the source and detector are contained within an imaginary plane which is typically substantially perpendicular to the patient's longitudinal axis. Radiation is directed through the patient, and the detector senses radiation which is not scattered or absorbed by the patient. The amount of radiation which reaches the detector is inversely related to the density of the "slice" of the patient being examined.
The irradiated slice creates a shadow or "projection" on the detector analogous to the pattern which is received and displayed by traditional X-ray film. Once the data for a given slice have been collected, the patient table is indexed to a new position and another slice is analyzed. The data captured by the detector or detector array are stored in memory, processed and "back projected" onto an "image matrix" to create a sectional density map of the irradiated slice.
Early medical CT scanners included a single X-ray source and a single detector. The source and detector were linearly traversed across the body under examination and then rotated a few degrees and linearly traversed again. Thus, the source created a plurality of substantially parallel rays for any given view. While projection of the parallel-ray data was relatively simple and fast, this type of machine was fairly slow primarily due to the laborious traverse and rotate method of collecting the data. Speed is of the essence in a CT medical scanner since even the slightest movement of the patient at any time during the scan will cause the image to blur. Even shallow breathing can seriously impair the CT analysis of a patient's torso, for example. Unfortunately, traverse and rotate scanners, so-called first generation machines, required several minutes to complete a scan sequence.
To improve the speed and accuracy of CT scanners, second generation machines included a linear array of detectors rather than a single detector. A single source was linearly traversed across the patient and the resulting parallel rays were detected by the linear detector array. Following such a traverse sequence, the detector array and the X-ray tube were rotated a degree or so and the traverse sequence was repeated. The detector data, derived from parallel rays, was readily back projected to form an image, and accuracy was improved. In addition, the speed of second generation machines was generally better than that of first generation machines because the linear detector array offered several angular views, in effect, at one time. Following a traverse sequence, the detector array and source could be rotated several degrees, perhaps six degrees for six detectors, to a new position. Also, second generation machines made more efficient use of the available X-ray energy. Although the accuracy of second generation machines was improved, speed still suffered, and movement by a patient, either voluntary or involuntary, resulted in a blurred image.
In the further quest of speed, third generation CT scanners employ a fan-shaped beam of radiation which irradiates a plurality of detectors simultaneously. See, for example, U.S. Pat. No. 3,881,110. Such third generation machines typically include a single X-ray tube and a curvilinear array of detectors. The detector array and the X-ray source are located on the same radius but on opposite sides of the patient. They rotate about the patient in synchrony, with the detectors receiving the portion of the fan-shaped beam which is not absorbed or scattered by the patient. In the earliest third generation machines, the raw data collected by the detectors were typically reorganized or "rebinned" into a format which approximated traverse and rotate data. These scanners have an inaccuracy in that the attenuation data required by a rotating fan beam scanner does not exactly duplicate the data of a traverse and rotate scanner. Rather, some approximation must be made in binning the fan beam data into parallel ray data.
Accuracy was improved in fan-beam machines through the use of an algorithm disclosed in the article Reconstruction from Divergent Ray Data, by A. V. Lakshaminarayanan in "Technical Report No. 92", State University of New York at Buffalo, Department of Computer Science, January, 1975.
In spite of the fact that the "traverse" aspect of CT scanners was eliminated by third generation geometries, further increases in speed were desired. Back projection of "divergent data" (data derived from a fan-beam geometry) is more complex than back projection of "parallel data" (data derived from a parallel-beam geometry). Back projection of divergent data therefore generally requires the computer within the CT scanner to perform a greater number of arithmetic operations.
In an attempt to further improve speed and accuracy of medical CT scanners, fourth generation CT scanners employ large arrays of completely stationary detectors. The detectors are typically evenly spaced in circular fashion about the patient table, such that the center of the circle is substantially coincident with the patient's longitudinal axis. And, only the X-ray source, coplanar with the ring of detectors, is rotated in fourth generation machines. Thus it is clear that fourth generation machines are at least conceptually mechanically faster than earlier generation machines, since only the relatively small X-ray tube is rotated. The rotation speed of the tube can be quite high without mechanically distorting the gantry. However, it is perceived that the computational speed of medical CT scanners, including fourth generation machines, can and should be further increased.
The earliest fourth generation CT scanners included a ring of detectors which completely circumscribed the patient. The speed of later fourth generation machines was further improved through the realization that 360 degrees of detectors was unnecessary. See U.S. Pat. No. 4,293,912, issued to Walters.
It is therefore apparent that the mechanical and computational speed of CT scanners has improved over the years. The increase in mechanical speed is particularly impressive when the original traverse and rotate technique is compared to fourth generation machines wherein the X-ray tube rapidly (e.g., in one second or less) rotates about the patient. However, it is perceived that an increase in computational speed is still needed. Further, it is desirable to reduce the overall number of interpolations which are performed within CT scanners to improve their resolution.
The present invention is directed to a back projector which in preferred embodiments improves the speed and accuracy of CT scanners regardless of their source/detector geometries. For example, third and fourth generation medical CT scanners can incorporate the back projector. A series of simple coordinate transformations are used in the back projection process rather than transcendental function computation, and avoiding the computation of transcendental functions reduces the back projector's complexity. Also, the need for spatial interpolations prior to back projection (for fan centering, for example) is eliminated, somewhat reducing pre-back projection computation. Additional advantageous features of the back projector of the present invention will be apparent to those skilled in the art.